1. Field of the Invention
The present invention relates to a small air-cooled two-stroke gasoline engine having a displacement of about 15 cc to about 35 cc which is preferably used in a small-sized hand-held working machine such as a brush cutter or chain saw. More particularly, it relates to a small air-cooled two-stroke gasoline engine which is designed so as to reduce noxious pollutants in an exhaust gas, in particular, total HC (THC) without impairing output power characteristics.
2. Description of the Prior Art
Recently, due to increased environmental awareness, even with respect to a small air-cooled two-cycle gasoline engine which is used in a hand-held working machine such as a brush cutter or chain saw, it has been strongly desired to render an exhaust gas discharged therefrom less pollutive by reducing noxious pollutants such as HC, CO and NOx in the exhaust gas. For example, according to the regulation of exhaust gas bill in the State of California, i.e., so-called CARB 1999, it is required to reduce CO, total HC (THC) and NOx contents of an exhaust gas to not higher than 130 g/bhp-h, 50 g/bhp-h and 4 g/bhp-h, respectively, from 1999 onward.
FIGS. 8 and 9 show an example of a conventional small air-cooled two-stroke gasoline engine which has been subject to a demand for reduction of noxious pollutants contained in an exhaust gas.
The illustrated internal combustion engine 1' is a Schnurle scavenging type small air-cooled two-stroke gasoline engine which is incorporated as a power source into a hand-held working machine such as a brush cutter or chain saw and whose displacement is about 23 cc. The internal combustion engine 1' comprises a cylinder 2' having a combustion chamber 5' equipped with a spark plug 15, a crank case 3 connected to the bottom of the cylinder 2', and a piston 4' fit-inserted in the cylinder 2'. In the cylinder 2', an intake port 7 connected to a carburetor (not shown) and an exhaust port 10' are formed so as to open oppositely at different levels, and a pair of scavenging ports 9' 9' ' are formed symmetrically with respect to the longitudinal sectional plane bisecting the exhaust port 10' and the intake port 7. Opening and closing of these ports 10', 7 and 9', 9' are effected by the reciprocating movement of the piston 4'.
As in a customary internal combustion engine, reciprocating motion of the piston 4' is converted into rotational motion of a crank shaft 12, on which a balance weight 14 is mounted, via a connecting rod 11, and the output power from the crank shaft 12 is utilized as a driving force of the hand-held working machine.
In the internal combustion engine 1', during a reciprocation, i.e., two strokes of the piston 4', steps of compression, combustion, intake, scavenging, expansion and exhaust are effected in a well-known manner as a consequence of the vertical reciprocation of the piston 4'. In the conventional engine 1', for example, as shown in the conceptional diagram of FIG. 6 (B), opening and closing of the exhaust port 10' and the scavenging ports 9', 9' by means of the piston 4' are timed, in view mainly of output power characteristics, such that the exhaust port 10' and the scavenging ports 9', 9' are open when the crank shaft 12 is within ranges covering an angle of 140 degrees and an angle of 107 degrees in terms of its crank angle, respectively, each of which centrally contains the bottom dead center (BDC). In other words, the exhaust port 10' and the scavenging ports 9', 9' are closed when the crank shaft 12 is outside the above respective ranges in terms of its crank angle.
As shown in FIG. 5 (B) which is an enlarged view of the combustion chamber 5' and its surroundings, the combustion chamber 5' is a squish dome type combustion chamber which comprises a substantially conical main surface 5a' and an annual skirt-like squish band 5b' gently sloping and having a relatively large band width .alpha.' (maximum width: 8 mm, minimum width: 3 mm). The combustion chamber 5' is equipped with a spark plug 15' in the conical surface opposite to the exhaust port 10' in such a manner that a spark point SP' of the spark plug 15' is located nearer to the exhaust port 10' than the center line C of the combustion chamber 5'.
Further, as shown in FIG. 5 (B), a distance L' between the top surface 4a' of the piston 4' and the upper edge 4b' of a groove for retaining the upper piston ring 21' of piston rings is about 2.5 mm, and each of the piston rings 21', 22' has a thickness d' of about 2.0 mm.
In the conventional small air-cooled two-stroke gasoline engine 1' as described above which is used in a portable working machine, a fresh gas mixture (air-fuel mixture) is in part directly swept toward an exhaust port 10' and discharged therefrom, so that a so-called "blow through" amount is undesirably large. This leads to unsatisfactory fuel consumption. Further, it is extremely difficult to reduce pollutants contained in an exhaust gas, in particular, THC. To date, there have not yet been developed any practically effective measures to cope with these problems.